Crystalline polypropylene is broadly used as general-purpose resin because of its relatively excellent rigidity, heat resistance and surface hardness. However, these properties possessed by crystalline polypropylene are still unsatisfactory as compared with those of ABS resins, polystyrene resins or various engineering plastics.
Isotactic polypropylene is a stereoregular polymer, and the solid thereof has a partially crystallized structure. Properties of a solid are known to be intimately related to the solid structure. That is, the above-described various properties generally depend on the degree of crystallization, and according as crystallinity increases, rigidity, heat resistance, and surface hardness are improved. From this point of view, many approaches to improve these properties have hitherto been made, such as a method for reducing the by-produced atactic component by, for example, improving the catalyst as disclosed in Kagaku Zokan 43, "Kobunshi no Characterization to Bussei", Kagaku Dojin (1970); a method for broadening a molecular weight distribution as disclosed in JP-A-59-172507 and JP-A-62-195007 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"); and a method using a nucleating agent, for example, an aluminum or sodium salt of an aromatic carboxylic acid (see JP-A-58-80329), an aromatic carboxylic acid, an aromatic phosphoric acid salt, a sorbitol derivative [see JP-B-55-12460 (the term "JP B" as used herein means an "examined published Japanese patent application") and JP-A-58-129036], and a specific high-molecular weight nucleating agent (see JP-A-60-139710 and JP-A 60-139731).
However, none of these known methods have achieved sufficient improvements in the properties.
On the other hand, polypropylene is most commonly formed into a desired shape after being once melted. Crystallinity changes depending on the processing conditions, and thereby varying the properties. More specifically, rigidity and the like can be improved by a method of reducing a rate of cooling for crystallization, a method in which a molded article is subjected to annealing at a high temperature below the melting point, or the like technique. However, these methods cause reduction in rate of processing or require equipment or energy for the heat treatment. Also, the improvements obtained are still insufficient.